The invention relates to a brake servo unit and is advantageously applicable to all motor vehicles whose brake circuit is equipped with a servo unit of this kind.
With the particular aim of improving streamlining, the design of modern vehicles tends to provide increasingly sloping engine bonnets, whereas the number of accessories disposed under the bonnet tends rather to increase. For this reason motor vehicle manufacturers impose maximum dimensions on some of these accessories. In particular, the outside diameter of brake servo units is generally limited by motor vehicle manufacturers to a certain value.
In order to obtain a sufficiently high brake boost power, especially for high-speed vehicles, manufacturers of brake servo units have therefore been led to design two-stage servo units having an increased effective section without any increase of the outside diameter of the servo unit. As an illustration, a servo unit of this kind is described in document GB-A-1 157 861.
As this document shows, these two-stage servo units are generally formed by placing end to end two booster chambers of approximately the same diameter. The limitation of the diameter of the servo unit then leads to an increase of its length, which is not always acceptable to the manufacturer.
Furthermore, the front wall of the casing of brake servo units is usually lined with a reinforcing plate. The number of plates constituting the frame of a two-stage servo unit and the number of fluidtight connections between these plates are therefore high, with a consequent increase of both production time and the cost of such servo units.
Accordingly, it has already been proposed, in the document GB-A-2,132,720, to construct a two-stage servo unit by arranging one servo unit within the other. Such a servo unit comprises two pistons disposed co-axially, each one being subjected to a pressure difference exerted on the diaphragm which is associated therewith, one of the diaphragms having an effective surface greater than that of the other. When the pistons are in their extreme operating position, the diaphragm of smaller diameter is partially received within that of larger diameter over a given length; this permits the reduction of the axial length of the two-stage servo unit by this given length, and thus correspondingly of the total volume of the servo unit, for a same booster force.
Nevertheless, this servo unit exhibits numerous disadvantages inherent in its tandem design. This design is similar to that of the two-stage servo units in which the two booster chambers are placed end to end. In fact, the two pistons are disposed end to end and are restored to their rest position by a spring disposed within the front chamber of the second servo unit, this spring acting on the second piston which, in its turn, acts on the first piston, itself acting on the actuating rod connected to the brake pedal. Applied to a servo unit in which the booster chambers are fitted into one another, this design inevitably leads to a reduction of the stroke of the pistons, and thus of the control and push rods; this is incompatible with certain harsh operating conditions.
Furthermore, with the arrangement of the pistons end to end, the reaction disc, in the rest position, is constantly subjected to the stress of the main restoring spring, and during operation it transmits the forces only between the first piston and the output rod and is insensitive to the action of the second piston. In order to remedy this disadvantage, it is then necessary to oversize the plunger and thus the control valve.